1. Field of the Invention
The present invention relates to a wide-angle lens system to be utilized in a single lens reflex (SLR) camera.
2. Description of the Related Art
In a single lens reflex (SLR) camera, since a mirror is provided between the photographing lens system and the image forming plane, there is a need to provide a back focal distance of a predetermined length to place the mirror therebetween. Accordingly, in a wide-angle lens system, a retrofocus lens system in which a back focal distance is longer than the focal length of the lens system has been generally employed. A retrofocus lens system is constituted by a negative first lens group and a positive second lens group, in this order from the object. Since the refractive power (hereinafter, power) distribution is non-symmetrical with respect to the diaphragm, spherical aberration, coma, distortion, astigmatism and the like tend to become large. In order to correct these aberrations, the number of lens elements has to be increased. However, in a retrofocus lens system, the entire lens system is arranged to be advanced along the optical axis in order to perform focusing. Therefore if the number of lens elements are increased, the weight of the lens groups to be moved is increased, which is undesirable for autofocusing. In particular, this problem has to be considered in a wide-angle lens system for medium and large sized cameras in which a brownie film is loaded.
Accordingly, if an attempt is made to materialize a camera with an autofocusing function, it is preferable to employ a lens system in which a part of the lens system is made moveable. Amongst such lens systems, a rear-focusing type lens system is preferable, since the entire length thereof is constant, and is easy to be handled. However, compared with a lens system in which the entire lens system is advanced, a rear-focusing type lens system has caused large fluctuations of aberrations upon focusing. This tendency is, in particular, noticeable in a retrofocus lens system.
It is an object of the present invention to provide a rear-focusing wide-angle lens system of a retrofocus type, which has high image-forming performance at most of photographing distances, and has a half angle-of-view of about 44xc2x0 and F-number of about 3.5.
In order to achieve the above-mentioned objects, there is provided a wide-angle lens system including a negative first lens group and a positive second lens group with a diaphragm, in this order from an object. Upon focusing, the positive second lens group is arranged to be moved along the optical axis. The wide-angle lens system satisfies the following conditions:
0.5 less than TL2/f less than 1.0xe2x80x83xe2x80x83(1) 
xe2x88x924 less than f1/f2 less than xe2x88x922xe2x80x83xe2x80x83(2) 
wherein
TL2 designates the distance, in the second lens group, between the most object-side surface of the most object-side lens element and the most image-side surface of the most image-side lens element;
f designates the focal length of the entire lens system when an object at an infinite distance is in an in-focus state;
f1 designates the focal length of the first lens group; and
f2 designates the focal length of the second lens group.
The wide-angle lens system according to the present invention preferably satisfies the following condition:
xe2x88x926 less than f1/f less than xe2x88x923xe2x80x83xe2x80x83(3) 
Furthermore, the wide-angle lens system according to the present invention can satisfy the following condition:
1.3 less than TL1/TL2 less than 2.8xe2x80x83xe2x80x83(4) 
wherein
TL1 designates the distance, in the first lens group, between the most object-side surface of the most object-side lens element and the most image-side surface of the most image-side lens element.
Still further, it is preferable to provide cemented lens elements in which the convex cemented surface faces towards the image, and the cemented surface satisfies the following condition:
0.2 less than (n1xe2x88x92n2)xc2x7f/Rc less than 1.0xe2x80x83xe2x80x83(5) 
wherein
n1 designates the refractive index of the object-side lens element with respect to the cemented surface; and
n2 designates the refractive index of the image-side lens element with respect to the cemented surface; and
Rc designates the radius of curvature of the cemented surface provided in the first lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-354772 (filed on Dec. 14, 1999) which is expressly incorporated herein by reference in its entirety.